Google researchers found a software flaw in several models of FireEye's security appliances that they say could give a cyberattacker full access to a company's network.

It's not unheard of to find security flaws in security software, but the latest discovery highlights once again how no technology is immune to such problems.

FireEye issued a statement on Tuesday saying it had issued a patch for the flaw, which affects its NX, EX, FX and AX Series appliances. The appliances passively monitor network traffic and pluck out suspicious files for study away from the live network.

The products have full access to a network, which means if attackers got a foothold in one, they could monitor and tamper with traffic or insert spying code.

The vulnerability is within a module that analyzes Java archive (JAR) files, FireEye said.

A successful attack could be triggered merely by sending someone an email or getting the person to click a link, wrote Tavis Ormandy of Google's Project Zero, who found the issue with Natalie Silvanovich. It's a "nightmare scenario," he wrote.

"This would mean an attacker would only have to send an email to a user to gain access to a persistent network tap -- the recipient wouldn’t even have to read the email, just receiving it would be enough," Ormandy wrote.

Once access has been gained to the FireEye appliance, it would be possible to load a rootkit, move around to other networks or install a self-propagating worm, Ormandy wrote.

Project Zero nicknamed the vulnerability "666." FireEye was notified on Friday and issued a temporary fix over the weekend and then a permanent one on Monday, according to its statement.